Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Equitare, Arcum tendere, Veritatem dicere —Herodotus
What's up with the epigraph?
The motto, in Latin, translates to: To ride, shoot straight, and tell the truth. It's Karen Blixen's personal motto, which is really not a bad way to live, especially if you're a farmer, hunter, and author. It's the classical Greek conception of the ideal life, so maybe the author is trying to live her life according to certain values.
And, maybe, she used those values to tell her story. Riding and shooting straight are important survival tools in Kenya—remember all those lions, hyenas and leopards? But telling the truth is not necessarily all that important, unless you want your readers to trust you.
The epigraph is like a mini-pact with the reader, promising to tell the truth. Bet you didn't know you'd be blood-siblings with Isak Dinesen just by opening her book. It could also be an aww-inspiring shout-out to Blixen's beloved Denys Finch-Hatton:
What [the people] really remembered in [Finch-Hatton] was his absolute lack of self-consciousness, or self-interest, an unconditional truthfulness which outside of him I have only met in idiots. (5.3.36)
This is a dude who lived (and, yes, died) by the principles of riding (or rather flying a freaking plane), shooting straight (with his gun, and his two tickets to the gun show) and telling the truth. Blixen loved this guy deeply, so it makes sense that she'd want to dedicate her memoir of Africa to him. Aw, shucks.